Electricity Producing Condensing Furnace

The Electricity Producing Condensing Furnace can be a cost-effective, fuel-saving, and lower carbon footprint replacement for natural gas and propane-fueled hot air furnaces.
Fundamental Theory and Practice
Gas-fueled hot air furnaces became a preferred home heating method after World War II when natural gas became available with long-distance pipelines and distribution systems.
The early gas hot air furnaces were thermostatically ON-OFF controlled with a pilot light, a circulating fan for space air, and a natural draft chimney. Elevated flue temperatures were required to exhaust the combustion products and prevent damaging acidic condensate to the chimney. Chimney energy loss also occurred when it was not hot operating. Efficiency which was defined as the fraction of the heat from combustion that was transferred to the heated space could range from 60% to 75%.
The “condensing furnace” that approached 100% efficiency became the next standard. An additional heat exchanger was added to extract heat from the combustion products down to near ambient temperature along with capturing the latent heat from condensing most of the water vapor. The chimney was replaced with a clothes drier-type vent to the side of the building and a condensate drain. Without the benefit of the buoyancy of a chimney, a small fan is used to force the exhaust of the combustion products.
It is noted that 100% efficiency is a 1st law efficiency but is not the best that can be achieved. The ideal efficiency would be an ideal fuel-burning engine driving an ideal or Carnot heat pump. An example would be a natural gas-fueled engine with a 25% efficiency with mechanical power driving a heat pump with a Coefficient of Performance of three and also recovery most of the 75% rejected engine heat for space heating. The resulting heating system would have a first law efficiency of 150%.
The subject Electricity Producing Condensing Furnace is a method of getting the same overall benefit, but with a module compatible with the connections of an existing condensing furnace. The fuel is supplied to a single cylinder air-cooled engine driving a grid-connected induction generator that converts 20% of the fuel to electricity while recovering the remaining 80% as space heat from engine cooling and exhaust.
Simplicity results from a single module serving as a furnace and electric generator. The generator also serves as the starting motor. The electric grid provides starting power and then voltage and speed control. The engine serves as the combustion chamber and high-temperature heat exchanger. The exhaust stroke of the engine provides the required forced purging of combustion gas. The spark plug serves as the ignitor. The engine cooling fan also circulates the space air.
The United States Department of Energy provided funding for the initial development and demonstration. It was also recognized as an Energy-Related Innovation of the Year by the American Society of Mechanical Engineers.
Initial Concerns and Resolution
While the potential of the Electricity Producing Condensing Furnace was recognized, there were significant concerns related to safety, maintenance, engine life, and noise. It was noted that each of these concerns has been resolved. The original team of Union College faculty and students built prototypes for demonstration and resolved some concerns.
Other concerns have been resolved by the introduction of a similar engine-driven home cogeneration system by Honda called Freewatt. The Honda system has more features that also add cost and complexity, such as the ability to operate separately from the electric grid.

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